Code Action:
*#61#- and tap Call To verify the number of unanswered calls
*#62#- and tap Call Use to verify the number for call forwarding
*#06# -To display the IMEI Number of the mobile
*777# -and tap Call To display the account balance information for prepaid iPhone
*225# -and tap Call To display the account balance information for Postpaid iPhone
*#67# -and tap Call To display the number for call forwarding when the iPhone is busy
*3001#12345#*- To display many of the inner settings of your iPhone
*#21# -To verify the call forwarding is enabled or disabled
*646# -and tap Call To display the Post-paid minute balance
*#33# -and tap Call To Check for call control bars.

> Enter field mode
3001#12345# and tap Call
Field mode reveals many of the inner settings of your iphone specially up-to-date network and cell information.

*#30# and tap Call. Calling line presentation check.
This displays whether you have enabled or disabled the presentation of the calling line, presumably the number of the party placing the call.

*&76# and tap Call. Check whether the connected line presentation is enabled or not.
State whether the connected line presentation is enabled or disabled.
Presumably similar to the calling line presentation.

*#61# and tap Call.
Check the number for unanswered calls.
Who the number for voice call forwarding when a call is answered.
Also show the options for data, fax, sms, sync, async, packet access and pad access

*#62# and tap Call.
Check the number for call forwarding if no service is available. Just like the previous, except for no-service rather than no-answer situations.

*#67# and tap Call.
Check the number for call forwarding when the iPhone is busy.
And again, but for when the iPhone is busy.

*#33# and tap Call. Check for call
control bars Here you will find all GSM-Codes/Secret-Code, for the iPhone are relevant.

*3001#12345#* To get your access to the field test mode of iPhone.

*#30# Thus, you can check whether the caller identification (Clip-function) in the iPhone is active.

Change PIN1: **04* old PIN * new
PIN * new PIN #
Change PIN2: **042*old PIN * new
PIN * new PIN #
*#746025625# SIM Clock Stop Mode
Whether your SIM card the SIM Clock Stop mode supported, you can find out with this code. The iPhone will then idle at the SIM card, is no calls, etc., all activities on the SIM card.
This has a positive effect on power consumption and prolongs battery life.